The atlantoaxial segment consists of the atlas (C1) and axis (C2) and forms a complex transitional structure bridging the occiput and cervical spine. The functional result of the joint is two-fold: (1) providing support for the occiput and (2) providing the greatest range of motion and flexibility possible while still maintaining stability. The instability in this joint is usually congenital, but in adults, it may be due to an acute traumatic event or degenerative disease.,,
The atlas is a ring-shaped structure that is devoid of a vertebral body, consisting of anterior and posterior arches bridged by two lateral masses. The lateral masses of C1 consist of superior and inferior articulating facets. The superior facets are medially angled, paired structures that articulate with the occipital condyles. The inferior facets articulate with the axis. Immediately posterior to the anterior arch of C1 is the odontoid process of C2. This configuration allows for a significant degree of flexion, extension, and rotation at the craniocervical junction. Several ligaments bridging the occipitoatlantoaxial complex produce the stability required to prevent devastating neurologic injury; the most important of which is the cruciate ligament. The cruciate ligament is composed of a transverse axial ligament that runs horizontally between the lateral masses of C1 (considered to the be the strongest ligamentous attachment in the cervical spine), posterior to the odontoid process, and a vertical ligament extending from the basion to the C2 vertebral body. Additional stability is provided by paired alar ligaments running from the tip of the odontoid process to the foramen magnum and the apical ligament connecting the tip of the odontoid process to the basion. Finally, anterior longitudinal ligament, rostral extension of the posterior longitudinal ligament (tectorial membrane), facet joint capsules, and the posterior myoligamentous complex complete the cohesive occipitoatlantoaxial junction. The list of common disorders associated with atlantoaxial instability includes the following:
In individuals without any predisposing factors, atlantoaxial instability is extremely rare.
Radiographic atlantoaxial instability is seen in up to 30% of patients with Down syndrome (DS), but only 1% of patients with DS have symptomatic atlantoaxial instability. Patients with rheumatoid arthritis (RA) are also susceptible to cervical spinal instability and atlantoaxial instability. Ranges of 25% to 80% have been seen, and newer disease-modifying RA drugs may change the course of the disease and decrease the incidence of atlantoaxial instability.
Atlantoaxial instability can occur equally in both genders and can present at any age. A high risk of atlantoaxial instability is seen in Down syndrome and older patients with rheumatoid arthritis.
Atlantoaxial instability can be classified into three generalized categories: inflammatory, congenital, and traumatic. RA is the most common inflammatory disease to affect the craniovertebral junction. This is due to a large number of synovial joints and ligaments contributing to the overall stability of the upper cervical spine. The chronic inflammation of RA leads to laxity and stretching of the transverse ligament, development of granulation tissue, and erosion of the bony structures and results in atlantoaxial subluxation and instability. Several congenital disorders are associated with atlantoaxial instability. The most well known of these is DS. Patients with DS are more prone to atlantoaxial instability due to ligamentous laxity and osseous abnormalities. Os odontoideum is the separation of the odontoid process from the body of C2. Previously it was thought to be a congenital lesion, but it is more likely due to a traumatic insult early in life. Patients with os odontoideum require dynamic studies to accurately demonstrate the degree of movement and canal compromise. Trauma as the sole cause of atlantoaxial instability is a unique entity and usually the result of a disruption of the transverse, alar, or apical ligaments. This type of injury is commonly associated with head trauma. Fractures of C1 or C2 also are traumatic causes of atlantoaxial instability.
Patients presenting with atlantoaxial instability can suffer from a wide variety and severity of complaints. Initial presentation can be with complaints of mild axial neck pain, paresthesias, weakness or signs of myelopathy on examination. Additional signs of lower cranial nerve dysfunction may also be seen in more advanced cases. Rarely, patients present with severe spinal cord injuries, causing quadriplegia or even death.
The "Rule of Spence" has classically determined stability of C1 fractures and is determined by measuring the lateral overhang of the lateral masses of C1 on C2 when viewing an AP radiograph, if the sum of both lateral masses of C1 on C2 is greater than 7mm the fracture is considered unstable; this measurement tool is also generally used in assessment of CT scan images. The atlantodens interval (ADI) is an additional method of determining atlantoaxial instability. The ADI is determined by measuring the distance between the posterior edge of the anterior arch of C1 and the anterior edge of the dens. A value less than three mm is considered normal in adults and a value less than 4.5 mm is considered normal in children. Numbers greater than the maximal normal values listed above suggest disruption of the transverse ligament. There are numerous proposed measurements for the assessment of atlantoaxial instability specific to patients with RA.
Treatment of atlantoaxial instability varies widely, and intervention is usually tailored to patients on an individualized basis. Asymptomatic patients can be monitored over time with dynamic imaging and MRI to monitor disease progression. Patients that develop symptoms or present with symptoms are usually encouraged to undergo surgical fixation and fusion to limit the chance of future neurologic injury. Nonsurgical intervention can consist of rigid cervical collar or halo immobilization. Surgical intervention may require, placing patients in traction preoperatively and both anterior and posterior approaches to the spine may be employed, depending on the patient's pathology.,,
Based on the staging system, the general treatment guidelines are as follows:
A four-stage classification scheme based on rotatory displacement has been developed:
Type III and type IV are highly unstable, and emergent treatment is recommended.
The prognosis in symptomatic patients who are treated is good. Posterior spinal fusion can help restore function and reverse symptoms like pain and myelopathy. However, in some patients, pain is a common symptom that can be disabling. Other individuals may develop the following neurological deficits:
In many cases, these neurological deficits are not always completely restored even with surgery.
The biggest risk of atlantoaxial instability is spinal cord compression which can present with the following complications:
Before any physical activity, a thorough neurological exam is recommended to determine the presence of any deficits. Premature exercise can result in paraplegia or quadriplegia.
Once the diagnosis of atlantoaxial instability is made, one should consult the neurologist, neurosurgeon, and a geneticist if the patient is a child.
The atlantoaxial segment consists of the atlas (C1) and axis (C2) and forms a complex transitional structure bridging the occiput and cervical spine.
The functional result of the joint is two-fold: to provide support for the occiput and the greatest range of motion and flexibility possible while still maintaining stability.
Instability in this joint is usually congenital, but in adults, it may be due to an acute traumatic event or degenerative disease.
Atlantoaxial instability can be classified into three generalized categories: inflammatory, congenital, and traumatic.
The "Rule of Spence" classically determines the stability of C1 fractures by measuring the lateral overhang of the lateral masses of C1 on C2 when viewing an AP radiograph. If the sum of both lateral masses of C1 on C2 is greater than 7mm, the fracture is considered unstable. This measurement tool is also generally used to assess CT scan images.
Treatment of atlantoaxial instability varies widely, and intervention is usually tailored to patients on an individualized basis. Asymptomatic patients can be monitored over time with dynamic imaging and MRI to monitor disease progression.
The prognosis in symptomatic patients who are treated is good. Posterior spinal fusion can help restore function and reverse symptoms like pain and myelopathy.